Hardcopy devices having a reciprocating head come in a variety of configurations, including inkjet printing mechanisms that use cartridges, often called "pens," which eject drops of liquid colorant, referred to generally herein as "ink," onto print media, such as a page of paper. Each pen has a printhead formed with very small nozzles through which the ink drops are fired. To print an image, the printhead is propelled through a printzone back and forth across the page, ejecting drops of ink in a desired pattern as it moves. By selectively energizing the resistors as the printhead moves across the page, the ink is expelled in a pattern on the print media to form a desired image (e.g., picture, chart or text). The nozzles are typically arranged in linear arrays usually located side-by-side on the printhead, parallel to one another, and perpendicular to the scanning direction of the printhead, with the length of the nozzle arrays defining a print swath or band. That is, if all the nozzles of one array were continually fired as the printhead made one complete traverse through the printzone, a band or swath of ink would appear on the sheet. The width of this band is known as the "swath height" of the pen, the maximum pattern of ink which can be laid down in a single pass. The print media, such as a sheet of paper, is moved through the printzone typically one swath width at a time, although some print schemes move the media incrementally by, for instance, halves or quarters of a swath width for each printhead pass to obtain a shingled drop placement which enhances the appearance of the final image.
The particular ink ejection mechanism within the printhead may take on a variety of different forms known to those skilled in the art, such as those using piezo-electric or thermal printhead technology. For instance, earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481. In a thermal system, a barrier layer containing ink channels and vaporization chambers is located between a nozzle orifice plate and a substrate layer. This substrate layer typically contains linear arrays of heater elements, such as resistors, which are energized to heat ink within the vaporization chambers. Upon heating, an ink droplet is ejected from a nozzle associated with the energized resistor.
As the inkjet industry investigates new printhead designs, one trend is toward using a "snapper" reservoir system where permanent or semi-permanent printheads are used and a reservoir carrying a fresh ink supply is snapped into place on the printhead. Another new design uses permanent or semi-permanent printheads in what is known in the industry as an "off-axis" printer. In an off-axis system, the printheads carry only a small ink supply across the printzone, with this supply being replenished through tubing that delivers ink from an "off-axis" stationary reservoir placed at a remote stationary location within the printer. Another inkjet system uses a "replaceable cartridge" where a disposable printhead is permanently attached to the ink supply, so when the ink reservoir is emptied, the entire cartridge including the printhead is replaced. A replaceable cartridge system assures the customer has a fresh, new printhead each time the ink supply is replaced. Some replaceable cartridges are monochrome (single color), for instance, carrying only black ink, while other cartridges are multi-color, typically carrying cyan, magenta and yellow inks. Some printing mechanisms use four monochrome cartridges, while others use a black monochrome cartridge in combination with a tri-color cartridge.
Whether the printing mechanism uses either a snapper cartridge system, an off-axis system, a replaceable cartridge system or some other inkjet system, drop placement on the media must be coordinated with the incremental advance of the media through the printzone for sharp, vivid images and text, which are free of print defects, such as color banding, improper spacing, printed line overlapping. Many types of inkjet printing mechanisms use a series of conventional paper drive rollers or tires to frictionally engage the print media and incrementally the media through the printzone, moving either a full or fractional swath width. To provide feedback to the printer controller regarding the location of the media with respect to the printhead, more recent printers, such as the DeskJet.RTM. 720C and 722C models of inkjet printers, manufactured by the present assignee, the Hewlett-Packard Company of Palo Alto, Calif., have incorporated an optical encoder wheel on the axle of the media advance tires. This system required two optical sensors to read the encoder wheel and correct for any eccentricity of the code wheel, as described in U.S. Pat. No. 5,774,074, which is assigned to the present assignee, the Hewlett-Packard Company. To determine the direction of rotation, one of these optical sensors was an expensive two-channel analog sensor that measured two signals which were in quadrature, that is, 90.degree. apart. These two signals were compared by the printer controller to determine which quadrature signal was rising first, and from this information, the direction of rotation was determined. Thus, it would be desirable to replace the most expensive one of these optical sensors with a more economical sensor, while also implementing a new printing routine that increases printing speed, to provide consumers with a faster, more economical printing unit.
Other hardcopy devices include scanners which have a scanhead with image receptors that "read" an image previously printed on media, and convert this image into an electronic file which may then be computer edited, or sent to a selected destination via either electronic mail (E-Mail) or facsimile transmitted over telephone lines, for instance. The image receptors in a scanhead may be a series of discrete elements arranged in a linear array, as described above for an inkjet printhead. These hardcopy scanning mechanisms may use the same media advance system as described above for an inkjet printing mechanism, and indeed, in many multi-function devices the same media advance system is used for both printing and scanning.
As a more general concept, both inkjet printheads and scanheads may be considered as "image transceiver heads," with printheads transceiving an image by printing that image on media, while scanheads transceive an image by "reading" an image that already exists on media. This generic image transceiver head may have one or more arrays of discrete interaction elements arranged, for instance, in a linear array, to selectively interact with media in an interaction zone of the hardcopy device. For a printing mechanism, the interaction elements are ink-ejecting nozzles and the interaction zone is a printzone. For a scanning mechanism, the interaction elements are image receptors and the interaction zone is a readzone, although in some multi-function devices, the printzone and readzone may both physically occupy the same location adjacent the media advance path.